Foil bearing

ABSTRACT

The present invention provides a foil bearing ( 1 ), comprising: a stationary mount member ( 2 ) surrounding an outer circumferential surface of a journal ( 13 ) of a rotating member via an annular gap (G); and a plurality of centripetal force producing foils (e.g., middle foils  12 ) arranged in a circumferential direction in the annular gap, where each of the centripetal force producing foils has one end fixed to the stationary mount member, wherein portions of the stationary mount member located between fixed ends of adjoining ones of the centripetal force producing foils are formed with an axially extending slit ( 4 ).

RELATED APPLICATION

The present application is a continuation application and claimspriority under 35 U.S.C § 120 to U.S. patent application Ser. No.10/946,331 filed on Sep. 22, 2004, the subject matter of which is herebyincorporated by reference in full.

TECHNICAL FIELD

The present invention relates to a foil bearing comprising a stationarymount member surrounding a journal of a rotating member via an annulargap and a foil assembly disposed in the gap to support the journal.

BACKGROUND OF THE INVENTION

It is conventionally known to use a foil bearing as a bearing for arotating member that rotates at a high speed such as at tens ofthousands rpm, in which the foil bearing comprises a plurality of foils(flexible membranes) for forming a bearing surface and supports ajournal (or shaft) of the rotating member by means of pressure of afluid dragged in between the journal and the foils as the rotatingmember rotates. Some of such foil bearings may comprise a plurality ofbump foils as disclosed in U.S. Pat. No. 4,277,113 issued to Heshmat, ormay have a plurality of leaf foils as disclosed in U.S. Pat. No.4,178,046 issued to Silver et al.

In these foil bearings, the bump foils or leaf foils are formed bypress-working a flexible thin metallic plate or the like so that theyprovide a resilient force for urging the journal generally in adirection toward a rotational center. If the foils arranged in thecircumferential direction have a uniform rigidity (or stiffness), thecenter of rigidity coincides with the geometric center of the bearing.In such a case, when the journal is rotating, the rotation center of thejournal in an equilibrium state is determined by an amount ofdeformation of the foils under a weight of the rotating member, andthus, if the rotation is clockwise, the rotation center will be shiftedin a lower left direction with respect to the center of the bearing atlow rotational speeds.

As the rotational speed of the journal increases, an air force actingupon the journal in an upper left direction becomes larger so that thecenter of the journal moves clockwise from the lower left shiftedposition toward the center of rigidity of the foils (or the center ofthe bearing). When the center of the journal approaches the center ofthe bearing, the air force acting upon the journal is reduced and thecenter of the journal moves in the lower left direction again. Theprocess is repeated at a frequency corresponding to the rotational speedto cause a whirling instability of the journal. In case of an air-filmfoil bearing, a force constraining the journal is relatively small, andthus, the above instability tends to become large particularly at aprimary bending resonance point, making it difficult for the rotatingmember to pass the resonance point safely as the rotation thereof isaccelerated.

In order to suppress such an instability and allow the rotating memberto pass the primary bending resonance point safely, it is necessary toreduce the amount of deformation of the foils due to the weight of therotating member, or, to provide the foils with appropriate rigidity andmake the foils exert an appropriate damping force. One way for that isto make the center of rigidity of the foils located at a higher positionthan the geometric center of the bearing by, for example, providing ahigher rigidity to the bump foils or leaf foils disposed at lowerpositions than those disposed at higher positions.

However, it is quite difficult to control with high precision therigidity or damping characteristics of a number of bump foils or leaffoils which are formed by press-working, and also, change in therigidity and damping characteristics of the foils requires considerablechange in the manufacturing process. Further, arranging the foils suchthat the foils at various circumferential positions have a suitablerigidity for their positions tends to result in a reduction in the yieldof the completed foil bearing which requires quite high precision invarious dimensions such as an inner diameter, cylindricity of the innerperipheral surface, etc. Due to these reasons, it has been quitedifficult to achieve the center of rigidity of the foil bearingpositioned higher than the geometric center of the bearing to reduce theoscillation at the primary bending resonance point and improve therotation performance of the foil bearing.

BRIEF SUMMARY OF THE INVENTION

In view of the above observations of the prior art, a primary object ofthe present invention is to provide a foil bearing in which the rigiditycan be easily adjusted to effectively reduce the oscillation at theprimary bending resonance point.

A second object of the present invention is to provide a foil bearingthat can make the center of rigidity of the bearing located at a higherposition than the geometric center of the bearing without the need forcomplicated manufacturing process.

A third object of the present invention is to provide a foil bearingthat can effectively provide a sufficient damping force for ensuring asteady rotation of the rotating member at high speeds.

In order to achieve such objects, the present invention provides a foilbearing (1), comprising: a stationary mount member (2) surrounding anouter circumferential surface of a journal (13) of a rotating member viaan annular gap (G); and a plurality of centripetal force producing foils(e.g., middle foils 12) arranged in a circumferential direction in theannular gap, where each of the centripetal force producing foils has oneend fixed to the stationary mount member, wherein portions of thestationary mount member located between fixed ends of adjoining ones ofthe centripetal force producing foils are formed with an axiallyextending slit (4).

The axially extending slits divide the circumferential wall of thestationary mount member into a plurality of circumferentially dividedparts (5), and it is possible to control an absolute value of therigidity of the stationary member as well as circumferential rigiditydistribution of the stationary mount member by adjusting the rigidity ofeach divided part for instance by varying the radial dimension(thickness) and/or circumferential dimension (angle or width) of eachdivided part. Because machining of the stationary mount member isrelatively easy, such adjustment of the rigidity of the divided partscan be achieved easily and with high precision. This can make itpossible to adjust the position of the center of rigidity of thestationary mount member (and hence of the foil assembly) as desired, towhereby improve the performance of the foil bearing without individuallycontrolling the properties, such as rigidity, of the foils attached tothe stationary mount member depending on the positions at which they aredisposed.

Preferably, the stationary mount member is provided with a thin portion(6) at either axial end thereof. This can widen the adjustable range ofthe rigidity of the stationary mount member.

According to another embodiment of the present invention, there isprovided a foil bearing, comprising: a stationary mount membersurrounding an outer circumferential surface of a journal of a rotatingmember via an annular gap; and a plurality of centripetal forceproducing foils arranged in a circumferential direction in the annulargap, where each of the centripetal force producing foils has one endfixed to the stationary mount member, wherein portions of the stationarymount member located between fixed ends of adjoining ones of thecentripetal force producing foils are formed with an axially extendingand inwardly facing groove (23) and wherein a plate member (25) isfitted in the groove, with axial ends of the plate member being fixed tothe stationary mount member such that an axially middle portion of theplate member does not contact a bottom of the groove.

According to such a structure, it is possible to arbitrarily set thecenter of rigidity of the stationary mount member by adjusting thematerial and/or thickness of the plate members, to whereby improve theperformance of the foil bearing easily.

Preferably, the bottom of each of the grooves of the stationary mountmember is formed with an opening (24). This ensures that the platemember does not contact the bottom of the groove of the stationary mountmember so that the plate member can slightly deform in the radialdirection.

In the above foil bearings, the centripetal force producing foilspreferably comprise a plurality of thin plates (12 a, 12 b, 12 c) eachhaving a base end fixed to an inner peripheral surface of the stationarymount member and extending in the circumferential direction, whereinfree end portions of the thin plates extending in oppositecircumferential direction overlap each other. In this way, when therotating member rotates and the thin plates (foils) oscillate, theyslide relative to each other to cause frictional damping (Coulombdamping), thereby achieving desired damping characteristics easily.Further preferably, the thin plates are provided with a coating, such ascopper coating, for controlling a frictional coefficient.

Thus, the present invention can provide an improved freedom ofadjustment of the center of rigidity of the foil bearing while achievinga sufficiently high precision required for the foil bearing, andtherefore is highly beneficial in improving the performance of the foilbearing.

Other and further objects, features and advantages of the invention willappear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

Now the present invention is described in the following with referenceto the appended drawings, in which:

FIG. 1 is a side view of a foil bearing according to one embodiment ofthe present invention;

FIG. 2 is a cross-sectional view taken along the line II-II in FIG. 1;

FIG. 3 is a cross-sectional view taken along the line III-III in FIG. 2;

FIG. 4 is a perspective view showing the foil bearing of FIG. 1;

FIG. 5 is a cross-sectional view similar to FIG. 2 and shows a foilbearing according to another embodiment of the present invention;

FIG. 6 is a perspective view of the foil bearing shown in FIG. 5, withpart thereof being cut away; and

FIG. 7 is a perspective view of essential part of the foil bearing shownin FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1-4 show an embodiment of a foil bearing according to the presentinvention. FIG. 1 is a side view thereof, FIG. 2 is a cross-sectionalview of an axially middle portion thereof taken along a lineperpendicular to the center axis, FIG. 3 is a cross-sectional view alongthe center axis, and FIG. 4 is a perspective view showing an outerappearance of the foil assembly. It should be noted that in order toclarify the relationship between the various parts, the parts in thedrawings may not be shown to the scale.

As shown in the drawings, a foil bearing 1 comprises a stationary mountmember 2 having a substantially cylindrical shape with a round innerperipheral surface, and a foil assembly 3 attached to the innerperipheral surface of the stationary mount member 2.

At positions dividing the circumferential wall of the stationary mountmember 2 into a plurality of equal parts (for example, into from eightparts to 32 parts), the stationary mount member 2 is formed with axialslits 4. The slits 4 are formed such that the axial ends of thestationary mount member 2 are left intact so as to preserve theintegrity of the cylindrical shape of the stationary mount member 2.

The slits 4 divides circumferential wall of the stationary mount member2 into a plurality of divided parts 5 which are each provided with athin portion 6 at either axial end. The thickness of the thin portion 6is selected such that the rigidity (or elastic deformability) of eachdivided part 5 allows an axially middle portion of each divided part 5to move slightly in a radial direction.

The rigidity of each divided part 5 can be arbitrarily adjusted asdesired by selecting the thickness of the thin portion 6 and/or radialand circumferential dimensions (or thickness and angle) of the dividedpart 5 itself. Thus, by providing a higher rigidity to the divided parts5 positioned on a lower side of the stationary mount member 2 than tothose positioned at an upper side, for example, it is possible to make alower portion of the circumferential wall of the stationary mount member2 have a higher rigidity (or be less deformable) than an upper portionof the same, whereby achieving a center of rigidity of the stationarymount member 2 positioned higher than a geometric center of the same.

The foil assembly 3 comprises a top foil 11 and a plurality of middlefoils 12 disposed outside of the top foil 11 and arranged in threelayers. A journal 13 of a rotating member, which has a substantiallyround contour in a cross-section perpendicular to the axial line, isinserted into a central portion of the stationary mount member 2 suchthat the inner peripheral surface of the stationary mount member 2 andthe outer peripheral surface of a journal 13 define an annular gap Gtherebetween, in which the foil assembly 3 is disposed.

The top foil 11 consists of a smooth sheet member curved in asubstantially cylindrical shape, and one end thereof is welded to aninner surface of one of the divided parts 5 positioned at a top portionof the stationary mount member 2 by a plurality of axially arrangedspot-welds 14 while the other end thereof extends clockwise to wraparound the journal 13.

The middle foils 12 each consist of a thin plate (or leaf piece). Morespecifically, the middle foils 12 comprise middle-layer leaf pieces 12a, inner-layer leaf pieces 12 b and outer-layer leaf pieces 12 c. Themiddle, inner, and outer-layer leaf foils 12 a, 12 b, 12 c can be formedby curving a smooth sheet member into a substantially cylindrical shapelike the top foil 11 and cutting it at appropriate angles, for example.Each leaf piece 12 a, 12 b, 12 c has a base end secured to an innersurface of an associated divided part 5 of the stationary mount member 2by spot welds 15, and the middle-layer leaf pieces 12 a extend clockwisefrom the base ends while the inner and outer-layer leaf pieces 12 b, 12c extend counterclockwise from the base ends. In this embodiment, threeleaf pieces 12 a, 12 b, 12 c, one from each layers, form a unit in whichthe leaf piece 12 a is sandwiched between the leaf pieces 12 b, 12 c andthe overlapping base portions thereof are bonded together by the spotwelds 15. As seen in FIG. 2, a plurality of such units of leaf piecesare arranged at an interval in the circumferential direction such thatthey are attached to one of every predetermined number (four, forexample) of divided parts 5 by means of spot-welds 15.

The plurality of units of leaf pieces 12 a, 12 b, 12 c are arranged suchthat the middle leaf piece 12 a of one unit slidably overlaps with theoppositely extending inner and outer leaf pieces 12 b, 12 c of anadjoining unit to form the three-layered structure. The leaf pieces 12a, 12 b, 12 c always exert a center-directed elastic force (orcentripetal force) upon the journal 13 and serves as centripetal forceproducing foils.

When the journal 13 oscillates, the oscillation is transmitted to thedivided parts 5, and the oscillation of the divided parts 5 causes theleaf pieces 12 a, 12 b, 12 c of the different layers to slide relativeto each other to thereby cause frictional damping (Coulomb damping). Inorder to create the frictional damping force effectively, both surfacesof each leaf piece 12 a, 12 b, 12 c as well as the outer circumferentialsurface of the top foil 11 are applied with a copper coating, forexample. The damping characteristics can be controlled as desired bychanging the material and/or thickness of the coating to adjust afriction coefficient or by increasing/decreasing the number of middlefoils 12 included in the foil assembly 3. The inner peripheral surfaceof the top foil 11 is preferably applied with a polytetrafluoroethylene(PTFE) coating or the like in order to reduce frictional rotationresistance at a low rotational speed.

FIGS. 5-7 show another embodiment of a foil bearing according to thepresent invention. In this embodiment, the foil assembly 3 issubstantially identical to that shown in the above described firstembodiment, and thus explanation thereof is omitted. In this embodiment,portions of a stationary mount member 21 opposing the elasticallydeformable portions of the middle foils 12, i.e., portions of thestationary mount member 21 between the fixed portions of the middlefoils 12, are formed with an axial groove 23 and an opening 24. Thestationary mount member 21 further comprises bridge members 25 each ofwhich consists of a rectangular metal plate (or strip) curved in anarcuate shape and are fitted in an associated one of the grooves 23 toextend axially across the opening 24. Axial ends of the bridge members25 are fixed to an inner periphery of the stationary mount member 21 bymeans of screws 26 or the like.

The weight of the journal 13 acts upon an axially middle portion of thebridge members 25 via the middle foils 12, and thus, the center ofrigidity of the bearing is determined by cooperation of the rigidity ofthe bridge members 25 and that of the middle foils 12. Therefore, inthis embodiment, it is possible to arbitrarily adjust the position ofthe center of rigidity of the bearing by suitably selecting the materialand/or thickness of the bridge members 25 and whereby providing eachbridge member 25 with appropriate rigidity. It should be noted that whatis essential is that the circumferential rigidity distribution of thestationary mount member 21 can be appropriately determined as desiredand therefore, for example, the openings 24 in the stationary mountmember 21 in the above embodiment may be replaced by axial grooves,which is deeper than the grooves 23, such that when the bridge members25 is fitted in the grooves 23, the bottoms of the deeper grooves arespaced apart from the bridge members 25.

As described above, according to the present invention, thecircumferential rigidity distribution of the stationary mount member 2,21 can be appropriately adjusted to set the center of rigidity of thebearing as desired without relying upon the manufacturing precision ofthe middle foils 12. Thus, as the rotational speed increases and thejournal 13 is lifted, the journal will reach an equilibrium positioncorresponding to the rotational speed and steadily rotates there with noor little whirling. In other words, in the foil bearing 1 according tothe present invention, the oscillation of the journal 13 at the primarybending resonance point can be considerably reduced so that the journal13 can pass the resonance point safely as the rotation is accelerated.

Although the present invention has been described in terms of preferredembodiments thereof, it will be obvious to a person skilled in the artthat various alterations and modifications are possible withoutdeparting from the scope of the present invention which is set forth inthe appended claims. For example, in the above embodiments, the middlefoils are arranged in three layers, but they may be in other number oflayer(s).

1. A foil bearing, comprising: a stationary mount member surrounding anouter circumferential surface of a journal of a rotating member via anannular gap; a plurality of centripetal force producing foils arrangedin a circumferential direction in the annular gap, where each of thecentripetal force producing foils has one end fixed to the stationarymount member, wherein portions of the stationary mount member locatedbetween fixed ends of adjoining ones of the centripetal force producingfoils are formed with an axially extending and inwardly facing grooveand wherein a plate member is fitted in the groove, with axial ends ofthe plate member being fixed to the stationary mount member such that anaxially middle portion of the plate member does not contact a bottom ofthe groove; and a top foil positioned in the annular gap between theouter surface of the journal and the plurality of centripetal forceproducing foils.
 2. A foil bearing according to claim 1, wherein thebottom of each of the grooves of the stationary mount member is formedwith an opening.
 3. A foil bearing according to claim 1, wherein thecentripetal force producing foils comprise a plurality of thin plateseach having a base end fixed to an inner peripheral surface of thestationary mount member and extending in the circumferential direction,wherein free end portions of the thin plates extending in oppositecircumferential directions overlap each other.
 4. A foil bearingaccording to claim 3, wherein the thin plates are provided with acoating for controlling a frictional coefficient.
 5. A foil bearingaccording to claim 1, wherein the axially extending groove radiallyextends through a portion of the stationary mount member.
 6. A foilbearing according to claim 2, wherein an axial length and an axialposition of the opening is substantially equal to an axial length and anaxial position of the centripetal force producing foils.
 7. A foilbearing according to claim 4, wherein the coating comprises copper.